Answer “what size air conditioner do I need?” with the sizing method the efficiency program actually publishes: the ENERGY STAR square-footage table, plus its documented adjustments for sun, occupancy, kitchens — and a ceiling-height correction for rooms taller than the table’s 8 ft assumption.
How to size a room AC
- Enter the room area (length × width; use the square footage calculator for L-shapes).
- Set the ceiling height — above 8 ft, capacity scales proportionally.
- Pick the sun exposure and the number of regular occupants; tick kitchen if it is one.
- Read the BTU/h figure and buy that size — not the next one up (see the FAQ on why).
The method, honestly
The base number is a published table lookup, not a formula: ENERGY STAR maps area bands to capacities (150–250 sq ft → 6,000 BTU/h · 300–350 → 8,000 · 450–550 → 12,000 · 700–1,000 → 18,000, and so on to 2,500 sq ft). On top go the same guidance’s adjustments — ±10% for sun, +600 BTU per extra person, +4,000 for kitchens. It’s deliberately a room-unit method: fast, sourced, and right for window and portable units. What it is not is a Manual J load calculation — insulation quality, window area, and climate zone are invisible to it, which is the trade-off for needing four inputs instead of forty.
Efficiency notes worth the minute
Match the unit to the window wiring and type (portables lose real capacity out the exhaust hose — their advertised BTU is now dual-rated under DOE rules, and the SACC figure is the honest one). Seal the window kit; a leaky install wastes more than a size step. And if the room runs hot because of sun, a $30 blackout curtain can beat a 1,000-BTU upgrade.
Private by design
The sizing runs on your device as you type — nothing about your home is uploaded or logged.